The present invention relates to portable screeds for use in leveling materials, such as loose gravel or wet concrete, and particularly to a material-leveling apparatus for use with a portable screed. More particularly, the invention relates to a material-leveling apparatus for use with a portable screed that includes an off-center weight rotating about a vertical axis so as to impart horizontal movement to the material-leveling apparatus.
After concrete is initially laid, it must be worked while it is wet to provide a smooth, homogenous mixture. Working the concrete helps settle the concrete and helps to densify and compact the concrete during finishing. The working also removes air voids and brings excess water and fine layers of aggregate to the surface for subsequent finishing.
A typical procedure employed in connection with the placing of concrete involves strike-off, bull float, as for rock washdown and, finally, hand-finishing, typically involving the use of trowels and screeds. A screed is a leveling device drawn over freshly poured concrete. The foregoing procedure is time consuming and, therefore, a need has arisen for more rapidly leveling freshly poured concrete with professional quality results.
Generally, the working of wet concrete is accomplished by using screeds having various types of leveling boards or "straight edges" such as floaters, curl edge boards, finishing boards, strike-off boards, and the like. The material-engaging portion of a straight edge is largely dictated by the intended use in working or leveling dry or wet materials. For example, "floaters" are designed to "float" on the wet concrete without any external support, such as might be provided by wood or metal forms. Therefore, floaters are an example of straight edges that require a wider base surface than other straight edges that rely on wood or metal forms for supporting freshly poured concrete.
Some conventional straight edges include a horizontal base portion for floating on the wet concrete and a vertical leading edge which is coupled to a support frame. Typically, the horizontal base portion extends rearwardly from the leading edge and the entire horizontal base portion lies in one plane. It is known to provide an integral, outwardly angled rear edge which projects upwardly away from contact with the concrete, as disclosed in U.S. Pat. No. 4,798,494 to Allen.
The frame is typically coupled to the leading edge or horizontal base portion of the material-engaging portion. In the case of a powered screed including a vibration head having a rotatable off-center weight, rotation of the off-center weight generates vibration and causes movement of a straight edge mounted on the vibration head in a horizontal plane. We have observed that the horizontal movement of the vibrating straight edge causes the straight edge to flex, which induces stress in the straight edge. In a floater-type straight edge having a wide horizontal base portion, the vibration-induced stress is particularly strong at the trailing edge. This vibration-induced stress can lead to cracking, fracturing, and/or tearing of the straight edge and, ultimately, complete failure of the straight edge. This vibration-induced failure shortens the useful life of a straight edge and leads to increased costs as a result of a need to replace failed straight edges periodically. A straight edge having a wide horizontal base portion, for use as a floater, that would allow vibrating movement of the horizontal base portion in a horizontal plane while limiting the amount of vibration-induced stress acting at the trailing edge would reduce the likelihood of straight edge stress fracture, and thereby provide a substantial improvement over conventional straight edges.
According to the present invention, an improved material-leveling apparatus is provided for use with a powered screed of the type including a frame supporting a vibration head. An off-center weight is coupled to the frame so that it is able to rotate about a vertical axis and impart a horizontal movement to a material-leveling apparatus carried on the vibration head to level the material in process contacted by the vibrating material-leveling apparatus.
The straight edge includes a vertical leading edge, a vertical trailing edge, and a horizontal base portion extending therebetween. The leading and trailing edges cooperate with the base portion to define a channel extending along the length of the straight edge. A bracket for rigidifying the straight edge is positioned in the longitudinally extending channel and coupled to the leading and trailing edges of the straight edge to stiffen the straight edge.
In preferred embodiments, the stiffening bracket includes a pair of foot portions, a central body portion, and a pair of angled leg portions interconnecting the foot portions and the central body portion. Illustratively, each stiffening bracket is shaped so that each of the foot portions are flat vertical plates that conform to and mate with the flat inside wall of the vertical trailing edge of the straight edge. Bolts or the like are used to couple these foot portions to the vertical trailing edge. The central body portion of each stiffening bracket is also a flat vertical plate that abuts the vertical leading edge of the straight edge.
By providing a vertical trailing edge on a straight edge and a rigidifying bracket in a channel formed in the straight edge, the present invention greatly reduces the occurrence of stress cracking in straight edges. Thus, the present invention provides an improved material-leveling apparatus and thereby reduces the labor and material costs associated with replacing cracked straight edges used in screeds.
Additional objects, features, and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of a preferred embodiment exemplifying the best mode of carrying out the invention as presently perceived.